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Al'IATEUR RADIO By GARRY CHATT, VK2YBX Understanding frequency modulation Despite being one of the most popular modes on the VHF & UHF bands, there are many who are still don't know how FM is generated. So let's take a look at the "nuts 'n bolts" of FM transmissions. The technique of frequency modulating a carrier is usually credited to Major Edwin Armstrong who, during the 1930s, experimented with various methods of reducing the noise that was associated with the reception of AM signals. He discovered that when the frequency of a carrier signal was varied according to the amplitude of the modulating signal, AUDIO AMPLIFIER wide bandwidth required for FM transmission; (2) the fact that best results are achieved using the groundwave mode of propagation (often called "line of sight"); and (3) the problem of phase distortion which occurs when FM signals are bounced off the ionosphere. In fact, operation in the VHF and UHF bands is quite an advantage, as an FM signal can be generated CLIPPER PHASE FILTER MODULATOR MULTIPLIERS PA at low frequency and then multiplied. For example, many crystallocked 2-metre transmitters use a fundamental oscillator at say 12MHz. This signal is then frequency modulated and then frequency multiplied 12 times (normally using two frequency doublers and a tripler) to achieve the output frequency (144MHz). In addition, once at the final frequency, the output stage need not be linear as would be needed for an AM or SSB transmission. Rather, it can be operated in class C for best efficiency. Fig.1 shows a block diagram of a typical FM transmitter. Frequency modulation CRYSTAL OSCILLATOR fig.1: block diagram of an FM transmitter. The signal from the phase modulator is frequency multiplied before it is fed to the power amplifier. a large improvement in received signal to noise ratio was achievable. By making a receiver that was able to respond to frequency and phase changes, but which remained insensitive to changes in amplitude, he was able to devise a mode of transmission with good immunity to noise (particularly impulse noise). This ability of FM (frequency modulation) to produce an excellent (by AM standards) signal to noise ratio, even at low signal levels, has now made it the preferred mode of communication for most utility and amateur operations. FM operations are largely limited to the UHF and VHF bands. There are several reasons for this: (1) the 84 SILICON CHIP (a) CARRIER I\ I\ I\ VV V I\ (b) MODULATING SIGNAL (C) MODULATED CARRIER Fig.2: in an FM signal, the carrier frequency increases during the positive half cycle of the modulating signal & decreases during the negative half cycle. Another major advantage that an FM transmitter has is that, unlike AM where a high level modulator is required (normally using a modulation transformer), it requires very little by way of a modulation stage. Normally two transistors (forming a simple amplifier) and a diode limiter are all that are required. This is due to the fact that as the signal is frequency multiplied, so is the level of modulation, or "deviation" as it is referred to in FM terms. Thus, a 2-metre transmitter requiring 5kHz of deviation at the output frequency only requires 1112th that level at the modulating stage. Because this level of deviation is so small, it is quite easy to make a simple FM modulator using a silicon diode as the active device. As the level of audio applied across the diode varies, so does its capacitance. When the diode forms part of the resonant circuit of an oscillator, the fFequency of oscilla- +1~~~~----,----,------,----,---.-------,--.-----r---r---, +0.81---41-------l---+--+---+--+---+-- -t---+---+--+----1 ffi +0 .6 i:: ~ ...... :5 +0.5 <-----<~.......+- c 6 C :a ~ ... +0.41----+l---+-,r,'----\-t->"'u--~......... +o.31-+-I---Alf---,/l\-------i-£\----+*-+*-::1:a¼4-'"'-:71""~~ C -0.4 ' - - - l - ------l---J.--"""':....::....---.1.._ 0 _ J _ _ - L -_ 7 _,__-'-_...i.....__ 10 _.____J 11 12 MODULATION INDEX (X) Fig.3: how the amplitudes of the carrier and sidebands vary with the modulation index. The first order sidebands are displaced from the carrier by an amount equal to the modulating frequency, the second set by an amount equal to twice the modulating frequency, & so on. tion will vary in direct proportion to the level of audio applied. This certainly simplifies construction of an FM transmitter! We know that it is possible to convey information by modulating a carrier either by varying the amplitude or frequency. It is also possible to modulate the carrier by varying its phase. Frequency and phase modulation are not independent of each other, as the frequency of the carrier cannot be varied without varying the phase. In an FM transmitter, the carrier frequency increases during the positive half cycle of the modulating signal and decreases during the negative half cycle. This change in the carrier frequency is called the deviation and is directly proportional to the instantaneous amplitude of the modulating signal. It therefore follows that the deviation is small when the amplitude of the modulating signal is small, and that maximum deviation occurs when the modulating signal is at a maximum. Fig.2 illustrates the nature of an FM signal. In a phase modulated (PM) signal, the deviation varies with both the instantaneous amplitude and the frequency of the modulating signal. Because the deviation increases with modulating frequency, this means that a PM transmitter has inbuilt pre-emphasis. This is the primary difference between FM and PM - in FM, the deviation is proportional to the instantaneous amplitude of the modulating signal only. A major difference between AM and FM is that an FM signal (and OEVIATION ~ v-1 -l-1 t I I I I I I I + FREQUENCY Fig.4: for slope detection, the carrier is tuned so that it sits on one side of the receiver's selectivity curve. As the signal swings between the deviation limits, it produces an AM output varying between X & Y. also PM signals) produce many sets of sidebands that occur at multiples of the modulating frequency on both sides of the carrier. Hence, an FM signal requires a wider bandwidth than an AM signal. The actual number of sidebands depends on the ratio between the modulating frequency, the frequency deviation and the modulation index. For sinewave modulation, the modulation index is the peak deviation divided by the modulating frequency (in Hz). For example, if a transmitter has a peak deviation of 3kHz either side of the carrier frequency, the modulation index when it is modulated by a sine wave of 1000Hz will be 3. If the same transmitter is now modulated by a lO0Hz signal, the modulation index changes to 30. By contrast, in a PM system, the modulation index is constant r egardless of the modulating frequency. In an FM system it varies with the modulating frequency. Also in an FM system, the ratio of the maximum carrier deviation to the highest modulating frequency is called the deviation ratio. Typically, the deviation ratio for commercial and amateur equipment operating at 5kHz deviation and with modulation limited to 3000Hz is 1.57. Fig.3 shows how the amplitudes of the carrier and sidebands vary with the modulation index. Assuming single sinewave modulation, the first order sidebands are displaced from the carrier by an amount equal to the modulating frequency, the second set is displaced by an amount equalling twice the modulating frequency, and so on. So it can be seen that the amplitude of the sidebands is dependent on the modulation index and not the amount of deviation. In addition, the carrier strength varies with the modulation index, unlike an AM signal where the carrier amplitude remains constant and the sidebands vary. At a modulation index of 2.405, the carrier disappears and at a higher index the phase is reversed, the energy being transferred from the carrier to the sidebands. However, the total power of the JUNE 1990 85 AMATEUR RADIO - UNDERSTANDING FM Vee Fig.5: block diagram of Motorola's MC3357 narrowband FM IC. It includes an oscillator, mixer, limiting amplifier, demodulator, active filter, squelch, scan control & mute switch all on one chip. signal remains the same, regardless of the value of the modulation index. This is why it is possible to use an inexpensive amplifier in the PA stage of an FM transmitter, as the amplifier does not need to be linear. Receiving FM & PM The reception of FM and PM signals requires a different type of receiver compared to that used in the reception of AM signals. First, the detector must be able to convert the incoming FM information 'into AM information. While FM signals can be received on an AM receiver (and vice versa), the results are unsatisfactory due to the high level of FROM IF AMPLIFIER ,- - -----, T1 distortion caused by the selectivity of the receiver. Fig.4 shows this effect. Hence an FM receiver must have a wideband IF filter. It must also have a limiter stage added between the last IF amplifier and the detector. This limiter is actually an overdriven amplifier. It is driven into clipping so that the output is at a constant level, regardless of input signal. As a typical input signal to a receiver can be in the order of fractions of a microvolt, it is necessary to have a significant amount of gain (often 50-60dB) before the limiter. This often requires six or eight cascaded stages of gain to ensure 01 1N34A 0.1 I I I I I I I I I I 100pF .001 I L_ RFC B+ 02 1N34A AUDIO GAIN VR1 500k .,. Fig.6: a practical FM disciminator circuit. Any shift in the frequency of the input signal causes a phase shift in the voltage components of the transformer secondary. These voltage components are then rectified and constitute the audio output. 86 SILICON CHIP stability. Each stage of gain must have a different time constant, with each stage limiting the range of signals passed by the previous stage. Fig.5 shows the block diagram of the MC3357 IC which is a complete FM limiter and discriminator. Note the limiter amplifier stage. Fig.6 shows a practical discriminator. The voltage induced in the transformer secondary is 90° out of phase with the input signal. As can be seen, the input signal is capacitively coupled to the secondary of the transformer. The secondary voltages appear so that one side leads the input signal, while the other side lags by an equal amount. When rectified, these signals cancel each other and produce zero voltage at the output. However, any shift in the frequency of the input signal will cause a phase shift in the secondary voltages at either end of the winding. The varying voltages (one increasing in output and the other decreasing) are then rectified and become the audio output (ie, they no longer cancel out). This output signal is fed to the following stage via a 0. lµF capacitor and 500k!1 pot. Disadvantages Although many consider the performance of an FM receiver to be superior to that of its AM counterpart, largely due to the FM receiver's ability to "capture" the strongest signal and produce a high signal to noise ratio, there is one significant advantage where safety is concerned in using an AM receiver. In an FM receiver, the strongest signal received, even if it is only two or three times stronger than other signals on the same frequency, will be the only signal demodulated. In an AM receiver, an S9 signal can be quite noticeably degraded by an S2 signal on the same frequency. This is one of the reasons why aircraft still use AM transmissions on VHF and UHF (so that it is always possible to hear both stations on the same frequency). This is despite the assumption of most that AM stands for "ancient modulation"! ~